Piezoelectric extension actuator

ABSTRACT

It is the object of the invention to create a piezoelectric expansion actuator for d33 piezoelements, which allows vibrations in structures to be suppressed. This object is achieved pursuant to a solution of the invention in that the expansion actuator ( 1 ) comprises a piezoelectric stack ( 2 ), which consists of d33 piezoelectric elements and is arranged between output elements ( 4 ), which are attached to the surface of the structure ( 7 ). The invention applies to a piezoelectric extension actuator, which is used to control vibrations in structures.  
     Another, alternative solution is based on the damping of vibrations between the main gearbox of a helicopter rotor and the cellular structure of the cockpit. The power application point of the output element ( 18, 19, 180, 190; 35, 36 ) is arranged at a distance from the corresponding end plate of the piezoelectric stack ( 22, 220; 31, 32, 33 ) in the axial direction (X).

[0001] The invention relates to a piezoelectric expansion actuatorpursuant to the preamble of Patent claim 1 and that of the Subsidiaryclaim 8.

[0002] The use of d31 piezoplates or d31 piezosegments is known for thepurpose of vibration control and to influence vibrations in structures.d31 piezoplates take advantage of the elastic transverse contraction ofthe piezoelectric material. Several piezoplates or piezosegments aredescribed in the following in summary as piezoelectric stacks. Apiezoelectric stack consists of several, but at least 2 piezoelements.With the above d31 piezoelements, for example, expansions are introducedinto carrier structures for helicopter transmissions so as to suppressthe transmission of body sound onto the helicopter cell. In doing so,the d31 piezoelements are integrated in accordance with their expansiondirection, which acts parallel to the surface of the d31 elements, intothe surface of the carrier structure across a large surface, e.g.through an adhesion technique.

[0003] By contrast, the expansion in the familiar d33 piezoelements actsperpendicular to the surface of the elements because d33 piezoplatestake advantage of the expansion of the piezoelectric material in thedirection of the applied field.

[0004] DE 198 13 959 A1 is aimed at making a device for body soundsuppression available that more effectively reduces the transmission ofequipment vibrations and oscillations through a carrier structure onto acellular structure of a cockpit in a simple construction and atrelatively low integration complexity. DE 198 13 959 A1 reveals that thedevice for body sound suppression comprises at least one piezo-actuator,which introduces the oscillations into the carrier structure in order toblock the body sound transmission path onto the insulating structuresubstantially and to compensate acoustic excitation by means of theexisting and excited system dimensions of the sound generator moreeffectively. This technical idea is not limited to use in helicoptermanufacturing. It can be employed in all areas of mechanical engineeringwhere a device for body sound suppression becomes necessary.

[0005] Contrary to other familiar expansion actuators, the piezoactuatorimplements the application of power onto the carrier structure no longerin points, but rather across a relatively large surface of the carrierstructure. The carrier structure can be arranged for example between themain gearbox of a rotor and a cellular structure of the cockpit of ahelicopter. In this case, the carrier structure would be one or morestruts (also called gear struts). The piezoactuator is largely arrangedalong the entire circumference of the strut and exhibits a definedexpansion in the axial direction of the strut. Forces are introduced bythe piezoactuator pursuant to DE 198 13 959 A1 via its surface.

[0006] The efficiency of power application is limited by the effectivesurface of the strut that is to be covered.

[0007] The invention is based on the task of creating a piezoelectricexpansion actuator for d33 piezoelements, with which vibrations can besuppressed in structures, and furthermore of considerably increasing theefficiency of power application of a piezoactuator despite the contrarytendency of decreasing construction volume of the piezoactuator.

[0008] This task is resolved pursuant to the invention with the featuresof patent claim 1 as well as alternatively with the features of thesubsidiary claim 8. Further developments of the invention are providedin the dependent claims.

[0009] A solution pursuant to the invention is based on the fact that ad33 piezoelement in the form of a stack is clamped into a mechanicalframe, which is fastened to the surface of the structure. Apart from ahighly specific, mechanical power, the invented expansion actuator alsoachieves good efficiency. Also beneficial is the application ofmechanical pre-stress that is integrated in the actuator and whichallows critical tension strain to be avoided for the piezoelements.Optionally, devices can be integrated in the frame with which strokespeed transformations or stiffness transformations can be beneficiallyachieved.

[0010] In another solution pursuant to the invention, the efficiency ofpower application for the piezo actuator can be improved by considerablyincreasing the distance between the resting areas of two output elementsof a piezoactuator and a corresponding end plate of the piezoelectricstack in the axial direction towards the strut end. The output elementsof the mechanical frame form the power transmission means from thepiezoactuator to the strut. The hereby considerably enlarged strutdistance between the resting areas of the two output elements exhibitsless stiffness, consequently leading to the fact that for the expansionof this strut section less force is sufficient than in a comparableconfiguration of a piezoactuator where the distance of the restingsurfaces of the output elements largely corresponds to the length of thepiezoelectric stack. The piezoelement also uses the d33 piezoelectricelements.

[0011] Based on the drawing, exemplary designs of the invention areexplained in more detail in the following. It shows:

[0012]FIG. 1 an expansion actuator in a sectional view,

[0013]FIG. 2 an expansion actuator with stroke speed transformation,

[0014]FIG. 3 an alternative version of an expansion actuator with strokespeed transformation,

[0015]FIG. 4 a diagram of a strut with axially spaced output elements ofa piezoactuator,

[0016]FIG. 5 a sectional view of a strut with collar-shaped outputelements of a piezo actuator, and

[0017]FIG. 6 an alternative design of the output element with recesses.

[0018] The expansion actuator 1 shown in FIG. 1 is rigidly fastened tothe surface of a structure 7 and consists of a d33 piezoelectric stack2, two end plates 3, two output elements 4 and a prestress element 6.

[0019] The d33 piezoelectric stack 2 is arranged in its mechanical framesuch that its expansion direction runs parallel to the surface of thestructure 6 in which the expansion actuator 1 transmits itspiezoelectrically generated expansions. The d33 piezoelectric stacktakes up ⅓ of the material volume of a d31 piezoelectric stack forequivalent active expansions.

[0020] In the design in FIG. 1 the mechanical frame is formed by the twooutput elements 4, which are rigidly attached to the structure 6. Toaccomplish this, the output elements 4 are attached in such a way to thesurface of the structure 7 that their output surface 5 is alignedparallel to the respective end plate 3 of the piezoelectric stack 2. Theoutput elements 4 can be fastened to the structure 6 by means offamiliar attachment techniques, for example by gluing.

[0021] The output elements 4 can be adjusted on their attachment surfaceto variously bent or plane structural surfaces. In the design shown thestructure is a pipe with a circular concave surface. The length of thepiezoactuator corresponds to the length of the strut section that issupposed to be expanded.

[0022] The piezoelectric stack 2 is seated between its two end plates 3and held in place with a prestress element 6 at mechanicalprecompression stress. Possible damaging tensile loads acting upon theexpansion actuator 1 are compensated with this precompression stress andcan thus have no effect on the piezoelectric stack 2.

[0023] The prestress element 6 can be implemented for example with oneor more mechanically acting tension springs—as indicated symbolically inthe design in FIG. 1. It is also possible, however, to design the endplates 3 as elastic plates and to insert the piezoelectric stack 2 withcompressed end plates 3 into the mechanical frame at precompressionstress.

[0024] The expansion actuator 1 with stroke speed transformation shownin FIG. 2 corresponds to the previously described design except for thefollowing deviations. The end plates 3 of the piezoelectric stack 2 arenot connected directly with the output surfaces 5 of the output elements4, but rather by means of an elastic pressure web 8, and the outputelements 4 comprise an inwardly one-sided open slot 9 that runs parallelto the surface of the structure 7. Furthermore, the two output elements4 are rigidly connected with each other by means of a non-expandingsupport bar 11, which engages in the free end 10 of the output elements4. In place of a support bar 11 two parallel support bars. 11 that arearranged on either side of the piezoelectric stack 2 can be used, as isrevealed in FIG. 2 with a support bar 11 shown in the drawing.

[0025] On each output element 4, the elastic pressure webs 8, slots 9and support bars 11 form three joints “a”, “b” and “c” about which thelever sections of the output elements 4 can rotate and generate a strokespeed transformation in the expansion actuator 1.

[0026]FIG. 3 depicts an expansion actuator 1 with stroke speedtransformation in an alternative design version compared to FIG. 2 ofthe output elements 4 and joints “a”, “b” and “c”. The effect of leversections about the joints “a”, “b” and “c” in principle corresponds tothe previously described design in FIG. 2.

[0027] The output elements 4 here are designed with a lever 12 that isseated in joint “a”. The joint “b”, in which the piezoelectric stack 2engages with an output web 14, is arranged on the lever 12 with a firstlever section 13 at a distance to joint “a”.

[0028] Joint “c” is arranged on the lever 12 with a second lever section15 at a distance to joint “b”. Joint “c” engages in the support bar 11.

[0029]FIG. 4 shows a diagrammatic image of a strut 16. The strut can be,for example, a steel pipe with a fastening loop that is welded onto eachend. Such a strut is used, for example, in a quadruple setting in orderto connect the main gearbox of the rotor of the helicopter with thecellular structure of the cockpit of the helicopter. The main gearbox ishereby located above the ceiling of the cellular structure of thecockpit. The two components are connected in 4 locations by a strut 16,respectively. The main gearbox of the rotor is one of the main sourcesof noise generated in the cockpit. Since the strut 16 is seated on theinterface between the main gearbox and the cellular structure, it isuseful if elastic dimensional changes are generated on the strut, whichcan largely compensate the forces introduced via the strut. This iseffected, for example, through a controlled dimensional change(expansion or contraction) of the strut 16 in the axial direction X. Thecontrolled elastic dimension change is implemented with thepiezoactuator 17, which initiates a dimensional change, particularly achange in length in the axial direction X in a certain section D of thestrut. In the strut 16 pursuant to FIG. 4 as well two output elements18, 19 are arranged per piezoelectric stack. The output elements 18, 19,however, are not connected directly behind the end plate 20, 21 of thepiezoelectric stack 22 with the surface of the strut 16, but the restingsurfaces 23, 24 of the output elements 18,19 are arranged at a distanceto the end plate 20, 21 of the piezoelectric stack 22 in the axialdirection X. The piezoelectric stack 22 does not have to rest directlyon the surface of the strut. The force that is generated by thepiezoelectric stack 22 is introduced into the strut 16 on the restingsurface 23, 24. This force effects an elastic dimensional change in asection D of the strut 16 between the two resting surfaces 23, 24.

[0030] The section D along the strut circumference comprises thecorresponding sectional spatial structure of the strut, in short calledsection D. The elastic dimensional change there compensates thevibration force in the strut 16, specifically in the area of theinterface of strut and cellular structure.

[0031] The piezoelectric expansion actuator 17 is formed by d33piezoelectric elements, which are arranged in a piezoelectric stack 22.The two ends of the piezoelectric stack 22 are limited by the end plates20, 21. The output elements 18, 19 are arranged on said end plates 20,21. The power application point of an output element 18,19 on the strut16 is arranged at a distance from the end plate 20, 21 in the axialdirection X towards the fastening loop 160, 161. Gaining such a distanceis associated with gaining a lever arm that engages on both sides of theend plates of the piezoelectric stack. One lever arm E, F each is formedby the output element 18, 19. The lever arms E, F increase the section Dby their length since originally section D corresponded only to thelength of the piezoelectric stack.

[0032] The tensile force, for example, that is generated in a selectionof the piezoelectric expansion actuator is introduced into the strut viathe output surface of the output element. The section D located betweenthe output surfaces 23, 24 of the strut 16 is thus exposed to acontrolled dimensional change in an axial direction X. This changerepresents an elastic dimensional change. Compared to the previouslydescribed solution, this alternative solution takes advantage of thelower rigidity of an enlarged strut section. This increases theefficiency of power application of a piezoactuator 17 considerably. Itis, therefore, possible to use a substantially smaller piezoelectricstack without having to accept an efficiency loss.

[0033] Multi-axis influencing of the dimensional change of the describedsection D of the strut 16 can be controlled as a function of thepiezoelectric stack's configuration along the circumference of thestrut.

[0034] The arrangement shown in FIG. 4 can be also designed in theinside of a tubular strut.

[0035] As FIG. 4 also shows, this elastic dimensional change of thesection D is controlled with a control unit 25. In the area of eachoutput element 18, 19, preferably in the vicinity of the resting surface23, 24, the control unit comprises a sensor 26, which determines asignal from the quantitative value of the existing vibrational forcesand supplies it to the control device. The control device 25 regulatesthe piezoelectric stack 22 such that a force is generated by thepiezoelectric stack and is introduced into the strut 16 via the outputelements 18, 19 so as to affect an elastic dimensional change of thestrut section D.

[0036] The distance D between the resting surfaces 23, 24 can beimplemented in a variably adjustable manner by designing at least onelever arm E, F such that it can be enlarged and reduced.

[0037] The above explanations apply similarly to the piezoactuator 170in FIG. 4 with the output elements 180, 190 and the piezoelectric stack220 with the end plates 200, 210.

[0038]FIG. 5 shows a sectional view of an design for a strut 30, whereinthe two fastening loops on the ends of the strut 30 are not depicted.

[0039]FIG. 5 depicts three piezoelectric stacks 31, 32, 33, which form apiezoactuator 34 along with the output elements 35, 36. Thesepiezoelectric stacks 31, 32, 33 are offset from each other, for example,by 120°. Each piezoelectric stack can be arranged at a distance from thesurface of the strut. The stacks, however, can also be arranged on thesurface of the strut 30. The first version is shown in the example.

[0040] The axial axis of the piezoelectric stack is aligned in thedirection of the axial axis X of the strut 30. Each piezoelectric stackis arranged between two output elements 35, 36. The two output elements35, 36 engaging one piezoelectric stack 31, 32, 33, respectively,contain each an annular socket 37, 38, which encloses the strut 30 in aninterlocking and non-positive manner along its circumferential surface.Extending from the annular socket 37, 38 the output element 35, 36 opensup in a bell-shaped manner like a collar, which is arranged at adistance from the strut starting from the edge of the annular socket toits annular edge. This shape is described as an annular collar 39, 40.On the edge 41 of the collar 39 rests one end of the piezoelectric stack31, 32, 33, respectively. The other end of the three piezoelectric stack31, 32, 33 is respectively located on the edge 42 of the collar 40.

[0041] The annular socket 37, 38 of the collar 39, 40 exhibitssufficient rigidity and firmness that corresponds to a resting surface370, 380 which is connected with the surface of the strut 30 in thecircumferential direction in an interlocking and non-positive manner.The forces generated by the piezoelectric stacks 31, 32, 33 areintroduced via the resting surfaces 370, 380. Such a design for anoutput element 35, 36 permits action in a variety of spatial axes. Hencemore variable design possibilities exist for introducing power into thestrut 30. The forces and bending moments that are introduced into thestrut 30 can be used to effect an excursion in the longitudinal (axial)direction, a lateral bending excursion in any random direction and alsotorsion of the strut 30.

[0042] This elastic dimensional change also affects a structural regionin the strut 30 along section D.

[0043] By using at least two piezoelectric stacks that are arrangedaround the strut, the strut can be displaced in the longitudinal andlateral directions through appropriate selection of the individualpiezoelectric stacks. A suitable control or regulating device is notshown in FIG. 5.

[0044] It is also possible to introduce torsional forces by insertingthe piezoelectric stack at an angle, i.e. a configuration of at leastone piezoelectric stack that is tilted in relation to the longitudinalaxis X of the strut 30.

[0045]FIG. 6 depicts another possible design of an output element. It isshown as a single output element 360 without strut and withoutpiezoelectric stack. The output element 360 is guided in the directionof the axial axis X of a strut and fastened to the surface of the strutby means of its annular socket 390. A collar 400 is incorporated on theannular socket. This collar 400 contains recesses 401 so that weight ofthe output element can be saved. The collar 400 includes for examplethree arms, which can be arranged at an angle of 120° in relation to oneanother. These three arms of the collar 400 are connected and limited attheir ends by a ring 420. This ring 420 forms the edge of the collar400. One end of a piezoelectric stack is arranged on the edge of thecollar, respectively.

[0046] Pursuant to another design (not shown), it is also possible todivide an output element 360 partially into sectional output elements.To accomplish this, sectional output elements are arranged in anon-positive manner with each other in one direction along thecircumference of a strut in segments and are connected. In the viewingdirection of the X-axis, an output element can be divided intoindividual (wedge-shaped) segments, which are arranged around theX-axis. The output element is thus composed in segments of sectionaloutput elements. The output element pursuant to FIG. 5, for example,could be composed of three sectional output elements in the case ofthree piezoelectric stacks. This configuration of sectional outputelements is an easy option for retrofitting a strut on the helicopterthat has already been installed.

[0047] Such a configuration makes it possible to reduce the vibrationsgenerated by the main gearbox in relation of the cellular structure ofthe cockpit efficiently for the pilot and noticeably for the passengers.

1. Piezoelectric expansion actuator for reducing vibrations instructures, wherein the expansion actuator (1) comprises a piezoelectricstack 2 that is made of d33 piezoelectric elements and is seated betweenoutput elements (4), which are attached to the surface of the structure(7).
 2. Expansion actuator pursuant to patent claim 1, whereinmechanical precompression stress is applied on the piezoelectric stack(2) by means of a prestress element (6).
 3. Expansion actuator pursuantto patent claim 2, wherein the prestress element (6) consists of one ormore mechanical tension springs.
 4. Expansion actuator pursuant topatent claim 3, wherein the prestress element (6) consists of elasticend plates (3), which limit the piezoelectric stack (2) that is insertedin the output elements (4) under pressure.
 5. Expansion actuatorpursuant to one of the patent claims 1 through 4, wherein in theexpansion actuator (1) a stroke speed transformation is integrated. 6.Expansion actuator pursuant to patent claim 5, wherein for the purposeof achieving the stroke speed transformation in the output elements (4)an inwardly open one-sided slot (9) that runs parallel to the surface ofthe structure (7) and an elastic pressure web (8), respectively, arearranged between the end plates (3) of the piezoelectric stack (2) andthe output surfaces (5) of the output elements (4).
 7. Expansionactuator pursuant to patent claim 5, wherein for the purpose ofachieving the stroke speed transformation the output elements (4),respectively, are designed to include an articulating lever (12),wherein behind a first lever section (13) the piezoelectric stack (2)with an output web (14) engages the lever (12) in an articulating mannerand wherein behind a second lever section (15) the support bar (11)engages the lever (12) in an articulating manner.
 8. Piezoelectricexpansion actuator for reducing vibrations in structures, wherein atleast one piezoactuator is arranged on a strut, which connects the maingearbox of a helicopter rotor with a cellular structure of the cockpit,and the piezoactuator introduces a controlled force in one section ofthe strut for the purpose of elastic dimensional change of this sectionof the strut, characterized in that the piezoactuator (17, 170; 34)comprises at least one piezoelectric stack (22, 220; 31, 32, 33) made ofd33 piezoelectric elements, which is seated between output elements (18,19, 180, 190; 35, 36) that are attached to the surface of the structure(16; 30), wherein the resting surfaces (23, 24, 230, 240; 370, 380) ofan output element are arranged on the strut at a distance in relation ofthe corresponding end plate of the piezoelectric stack (22, 220; 31, 32,33) in the axial direction (X).
 9. Expansion actuator pursuant to patentclaim 8, wherein the output element (18, 19, 180, 190; 35, 36) forms alever arm (E, F) from its configuration on an end plate up to the powerapplication point.
 10. Expansion actuator pursuant to patent claim 8,wherein the distance (D) of the resting surfaces (23, 24, 230, 240) ofthe output elements (18, 19, 180, 190) can be adjusted variably. 11.Expansion actuator pursuant to one of the claims 8, 9 or 10, wherein thecollar (400) of the output element (360) contains recesses (401). 12.Expansion actuator pursuant to one of the claims 8, 9, 10 or 11, whereinthe output element can be composed of sectional output elements in acircumferential direction of the strut so that retrofitting of analready installed strut is possible.